Carrying capacity is the recognition that every ecology can hold only so much of certain things before something else degrades. A family can absorb only so much caregiving before caregivers collapse; a city only so much traffic before mobility falls; a river only so much contamination; an institution only so much administrative complexity before traversability collapses; a participant only so much responsibility before competence declines.
The point the model presses is that these constraints are not all physical. Alongside material capacity there is attention capacity, care capacity, administrative capacity, trust capacity, informational capacity, leadership capacity, and social tolerance — most of which no balance sheet records. This is exactly why ecological capacity is a richer measure than economic productivity: most of the ways a field runs out of room are invisible to money, and it is the limit that renewability exists to protect. See How a Living Field Works.